Anaerobic Digesters

The concept of the UASB reactor consists of a vessel in which the wastewater flows upward through an anaerobic sludge bed comprised of self-immobilized microbial communities.

The wastewater is fed into the bottom of the reactor through distribution pipes that extend throughout the width of the reactor. As the wastewater passes through the sludge bed, it comes in contact with the microorganisms and anaerobic degradation occurs.

The treated effluent leaves through an outlet at the top of the reactor. The produced biogas causes hydraulic turbulence as it moves upward through the reactor, providing adequate mixing within the system and eliminating the need for mechanical mixing. Granule (biomass) retention is facilitated by the presence of a three-phase separator (solid-liquid-gas separator) specially designed at the top of the reactor, where the water phase is separated from sludge solids and gas.

The range of wastewaters successfully treated using UASB technology is constantly expanding to include, not only high-strength industrial wastewaters, but also recalcitrant, toxic and low-strength wastes. The high biomass concentration within the UASB reactor results in rapid transformation of the contaminant to biogas, allowing for the application of high loading rates due to its design with a highly active biomass concentration. In other words, the soluble Chemical Oxygen Demand (COD) is readily converted to biogas, which is composed mainly by methane (65-70%) and carbon dioxide (30-35%) The produced biogas exits through a manifold and into a main line where it is collected for recovery or for burning.

The chemical reaction process stages passing through the ANAEROBIC System are:

First, complex organic materials are subjected to hydrolysis of lipids, cellulose and proteins where materials are hydrolyzed and fermented by facultative (those that live either in the presence or in the absence of oxygen) and anaerobic microorganism into fatty acids. Second, the acid genesis fatty acids (large chains as propionic, acetic and butyric acid) are oxidized by β-oxidation to produce Hydrogen (H2) and acetate (processes termed acetogenesis and dehydrogenation respectively). The last stage is methanogenesis which combines the Hydrogen (H2) with carbon dioxide (CO2) to form methane and also splits acetic acid into carbon dioxide (CO2) and methane (CH4).

This decomposition is performed by several facultative and anaerobic bacteria such as Clostridium, Bifidobacterium, Desulphovibrio, Actinomyces, and Staphylococcus. Finally, methanogenic bacteria such as Methanobacterium, Methanobacillus, Methanococcus, and Methanosarcina digest these fatty acids, resulting in the formation of methane gas.

The production of methane gas is the slowest and most sensitive step of the anaerobic digestion process because it requires specific environmental conditions for the growth of methanogenic bacteria. These bacteria can only digest effectively at a pH of 6.6-7.6, and if the growth of the acid forming bacteria is excessive, there will be an overproduction of acid leading to a decrease in the pH level which can cause many problems. Also, the methanogenic bacteria have a limited temperature range for optimum performance, usually in the mesophilic range (90 – 105 °F). Often this requires pre-heating of the wastewater before entering the anaerobic digester.